Traditionally, control of internal combustion engines has been based on the sensing of variables such as engine speed, intake manifold pressure, exhaust oxygen concentration, coolant temperature etc. and using these variables to adjust variables such as spark timing, exhaust gas recirculation rate, EGR, and fuel flow to a baseline engine condition that is measured on a test engine.
This approach has several drawbacks. Firstly, an engine will diverge from the baseline test engine due to production variation and component wear. Secondly, cylinder-to-cylinder variation may be significant. And thirdly, it appears that future engine combustion systems may render the traditional control approach inadequate.
An alternative approach is to implement a control system with the capability to adjust for changes in the individual engine cylinder operating characteristics. Such a control system is possible using cylinder pressure sensors and applying feedback control to ignition timing, dilution gas rate and fuel rate.
In a typical control system, there are three controlled parameters: spark timing (or fuel injection timing in a diesel engine), EGR rate and air/fuel ratio. The first parameter controls the timing of the ignition process and the other two parameters affect the speed and duration of the combustion process.
U.S. Pat. No. 4,622,939 (Matekunas et. al.) describes a control system for an internal combustion engine that uses pressure ratio management. The ratio of measured combustion chamber pressure to an estimated motoring pressure (i.e. the pressure within the cylinder when no fuel is being injected) is determined for a number of predetermined crankshaft rotational angles. These pressure ratios are used to control ignition timing for MBT (minimum ignition advance for best torque), EGR and fuel balance among combustion chambers.
Cylinder pressure within the Matekunas disclosure is determined via a pressure sensing transducer that produces a voltage that is linearly related to pressure. The voltage output signal of the transducer, Et, is related to the pressure, P, by the following relationship:Et(θ)=GP(θ)+Ebias  [1]where G is the gain of the transducer which is assumed to be constant for a given engine cycle and Ebias is a voltage signal offset such that Et−Ebias=0 when Pcyl=0, Pcyl being the absolute cylinder pressure.
It is assumed that prior to start of combustion the cylinder contents follow a polytropic process so that:PVn=constant  [2]where P is the pressure, V is the volume of the cylinder and n is the polytropic exponent.
The Matekunas disclosure derives, from equations 1 and 2, an equation for Ebias that uses the pressure transducer signal sampled at two crank angle points during the compression stroke (but prior to the start of combustion) along with a specified value for the constant n. It is noted that the polytropic constant is assumed to be constant over the sampling interval and that a value for n is accurately known in advance. Specifically, Ebias is calculated using the following equations:Ebias=[Et(θ1)−K2Et(θ2)]/(1.0−K2)  [3]K2=[V(θ1)/V(θ2)]n  [4]
During combustion the motoring pressure values, which are required to calculate pressure ratio, cannot be measured, but can be estimated using the polytropic relation, equation 2. Normally the same value of the polytropic constant used to calculate Ebias is assumed. Pressure ratios thus calculated may be used to estimate several combustion related parameters, including combustion timing, duration and dilution level.
Upon the application of the teachings of U.S. Pat. No. 4,622,939 to diesel engines a number of disadvantages become apparent. Firstly, the thermodynamic properties of the working fluid during the expansion stroke of a diesel engine are significantly different from those during compression. This degrades the accuracy of the estimated motoring pressure during expansion.
Secondly, since diesel engines have higher rates of change of pressure, it becomes more important to synchronize cylinder volume with the pressure signal. It is noted that the polytropic relation, equation 2, will give accurate results only if the cylinder volume is correct. Cylinder volume may be calculated as a function of slider-crank geometry, compression ratio and crankshaft position. There is usually significant uncertainty in compression ratio and crank position, so engine control accuracy may be improved if the control algorithm can learn correct values.
Thirdly, compression temperatures within diesel engines are high (as a result of the high compression ratios). Error in the estimated motoring pressure is therefore caused by (a) heat transfer losses and (b) decreasing ratio of specific heats with increasing temperature.
It is therefore an object of the present invention to provide a control system, controller and associated control method that substantially overcomes or mitigates the above mentioned problems.
According to a first aspect of the present invention, there is provided a method of finding a voltage offset of a transducer used to measure pressure within an engine cylinder, the transducer being arranged to output a voltage signal Et(θ) and having a voltage signal offset value Ebias at zero cylinder pressure and the contents of the engine cylinder undergoing a polytropic process, the method being comprised of the following steps;
a) measuring voltage output from the pressure transducer at least two crank angle values during the compression stroke;
b) calculating the volume of the cylinder at the crank positions where the voltage signals are measured;
c) calculating the ratio of specific heats for the cylinder contents;
d) using the values from (a), (b) and (c) to derive a value for the voltage signal offset Ebias.
The method according to the first aspect of the present invention provides a way of pegging a pressure transducer to find the voltage offset signal, Ebias, such that E−Ebias=0 at Pcyl=0, where E=pressure transducer voltage output and Pcyl=absolute cylinder pressure. In other words, the method allows recorded pressure data to be pegged (calibrated) to absolute cylinder pressure.
Conveniently the compression process may be modelled as a polytropic process, so the pressure, P, and volume, V, within the cylinder may be related by PVn=constant, where n is the polytropic constant. The transducer output Et(θ) may be defined by the relationship Et(θ)=G P(θ)+Ebias, where G is the gain of the transducer, P(θ) is the pressure within the cylinder at a crank angle θ and Ebias is the voltage signal offset value. Using the results of steps (a), (b) and (c), these relations may be used to solve for Ebias. (Note: as used herein, the terms polytropic constant and polytropic exponent are interchangeable).
Conveniently, the cylinder may comprise a piston arranged for reciprocal motion and the measuring step of the method comprises measuring the voltage signal outputs during a crank angle window of 90 to 60 degrees before top dead centre of the piston cylinder.
Preferably, the ratio of specific heats is calculated during the above mentioned crank angle window as a function of gas temperature and composition, based on a model of the engine system, the model comprising estimates for gas temperature and composition.
Conveniently, the value of Ebias may be derived according to the following equation:Ebias=[Et(θ1)−K2Et(θ2)]/(1.0−K2)wherein K2=[V(θ1)/V(θ2)]k, θ1 and θ2 are first and second crank angles, k is the ratio of specific heats calculated in step (c), V(θ) is the cylinder volume at crank angle θ and Et(θ) is the transducer output signal at crank angle θ. The biased voltage signal, E, given by E=Et(θ)−Ebias is henceforth used whenever a pressure or pressure ratio value is required.
According to a second aspect of the present invention, there is provided a method of correcting phasing errors between a voltage signal output of a pressure transducer used to measure pressure within an engine cylinder and the position of an engine crankshaft within an engine system, the contents of the engine cylinder undergoing a polytropic process such that PVn=constant, where P=cylinder pressure, V=volume of the engine cylinder and n=polytropic constant, the method comprising:
a) calculating the ratio of specific heats for the engine cylinder contents;
b) measuring the pressure within the engine cylinder and calculating the volume of the cylinder for at least two different crankshaft positions during an expansion stroke;
c) calculating a value for the polytropic exponent, n, from the equation PVn=constant using the values derived in step (b);
d) iteratively finding a crank angle phasing such that the value of n calculated in step (c) equals the ratio of specific heats calculated in step (a).
Preferably, the pressure measured in the measurement step is measured for a motoring engine, that is, when fuel is cut off during deceleration.
Preferably, the pressure measurement and volume calculation in step (b) are performed during a crank angle interval from 60 to 90 degrees after top dead centre.
Conveniently, n may be calculated from the following equation:n=(log E60−log E90)/(log V90−log V60)where E60, E90 are the biased voltage output from the transducer and V60, V90=cylinder volume at 60 and 90 degrees after top dead centre respectively.
According to a third aspect of the present invention, there is provided a method of determining the compression ratio of an engine, the method comprising:
a) measuring the pressure ratio of a cylinder within the engine near the end of an expansion stroke in order to derive a final pressure ratio, PRF;
b) calculating the pressure ratio of the cylinder at top dead centre;
c) varying the compression ratio of the engine used in the calculation of step (b) until the pressure ratio at top dead centre, PR(TDC), is a target fraction of the final pressure ratio.
Preferably, the pressure ratios calculated in steps (a) and (c) are based on cylinder pressure measurements on a motored engine.
Preferably, the final pressure ratio is derived by averaging the calculated pressure ratios over a crank angle interval from 60 to 90 degrees after top dead centre.
Conveniently, the compression ratio is varied as in step (c) untilPR(TDC)=Target PR(TDC) andTarget PR(TDC)=1−X(1−PRF) where X is the target fraction.
According to a fourth aspect of the present invention, there is provided a method of improving the accuracy of the calculation of heat release fraction for a cylinder in a firing engine, the contents of the engine cylinder undergoing a polytropic process such that PVn=constant, where P=cylinder pressure, V=volume of the engine cylinder and n=polytropic constant and the method comprising the steps of:
a) calculating the expansion polytropic exponent, poly_exp, for the firing engine;
b) calculating the compression polytropic exponent, poly_comp;
c) calculating an estimated motoring pressure using the polytropic relation, PVn=constant, with polytropic exponents determined in step (a) for crank angle values after-top-centre, and in step (b) for crank angles before-top-centre;
d) calculating pressure ratio given by PR=(measured pressure)/(estimated motoring pressure), using estimated motoring pressures calculated in step (c);
e) calculating the final pressure ratio, PRF, by averaging pressure ratio values late in the expansion stroke;
f) calculating heat release fraction, HRF, according toHRF=(PR−1)/(PRF−1)
The calculation of poly_exp in step (a) and PRF in step (e) are performed by averaging over a crank angle interval that begins after combustion is complete, and ends before the exhaust valve opens.
The value for poly_comp in step (b) is set equal to the value of the ratio of specific heats calculated as described in the first aspect of the invention.
According to a fifth aspect of the present invention, there is provided a method of calculating the heat release fraction for a cylinder in a firing engine, the method comprising:
a) calculating the motoring pressure ratio, PR_mot, of the engine according to the equation: PR=measured motored pressure (θ)/estimated motored pressure (θ), where θ is the crank angle and the estimated motored pressure being derived from PVn=constant, where P=cylinder pressure, V=cylinder volume and n=polytropic exponent, n being set equal to the ratio of specific heats of the contents of the cylinder.
b) calculating the pressure ratio of the motoring engine at the end of an expansion stroke, PRF_mot;
c) calculating the heat release fraction according to:HRF=(PR—cor−1)/(PRF—cor−1)                where PR_cor=PR/PR_mot, PRF_cor−PRF/PRF_mot and PR is the ratio of measured firing cylinder pressure to estimated motoring pressure and the final pressure ratio PRF is evaluated after combustion is complete.        
The method according to the fifth aspect of the present invention provides a method of calculating the heat release fraction for a cylinder in a firing engine that reduces the error due to heat transfer losses.
According to a sixth aspect of the present invention, there is provided a carrier medium for carrying a computer readable code for controlling a controller or engine control unit to carry out the methods of any of the first, second, third, fourth or fifth aspects of the invention.
The seventh, eighth and ninth aspects of the invention relate to apparatus suitable for carrying out the methods of the first, second and third aspects of the invention respectively.
According to a seventh aspect of the present invention, there is provided a device for pegging, or finding the voltage offset, Ebias, of a transducer used to measure pressure within an engine cylinder, the transducer being arranged to output a voltage signal Et(θ) and having a voltage signal offset value Ebias at zero cylinder pressure and the cylinder contents undergoing a polytropic process, the device comprising:
input means for receiving at least two measured voltage signal outputs from the transducer;
Processing means arranged to calculate the ratio of specific heats for the cylinder contents; calculate the volume of the cylinder at the points the voltage signals are measured and to subsequently derive a value for the voltage signal offset Ebias.
According to an eighth aspect of the present invention, there is provided a device for correcting phasing errors between a voltage signal output of a pressure transducer used to measure pressure within an engine cylinder and the position of an engine crankshaft within an engine system, the contents of the engine cylinder undergoing a polytropic process such that pVn=constant, where P=cylinder pressure, V=volume of the engine cylinder and n=polytropic constant, the device comprising:
input means for receiving at least two measured voltage signal outputs from the transducer;
processing means arranged to a) calculate the ratio of specific heats for the cylinder contents; b) calculate the volume of the cylinder from an engine model for at least two different crankshaft positions; c) calculate a value for the polytropic exponent, n, from the equation PVn=constant using the values of V derived in (b); and d) iteratively vary the phasing until the value for n calculated in (c) equals the ratio of specific heats calculated in (a).
According to an ninth aspect of the present invention, there is provided a device for determining the compression ratio of an engine comprising:
input means for receiving data related to the pressure ratio of a cylinder near the end of an expansion stroke;
processing means arranged to derive a final pressure ratio, PRF, from data received by the input means; calculate the pressure ratio of the cylinder at top dead centre; and to vary the compression ratio of the engine used in the calculation of pressure ratio at top dead centre until the pressure ratio at top dead centre, PR(TDC), is a target fraction of the final pressure ratio.
The invention extends to an engine control unit for a vehicle and a vehicle comprising a controller according to the first to fifth aspects of the present invention. The invention further extends to an apparatus corresponding to the fourth and fifth aspects of the present invention.